<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>
        /*body总有一个默认的外边距margin：0， 常见操作*/
        h1,ul,li,a,body{
            margin: 0;
            padding: 0;
            text-decoration: none;
        }

        /* border: 粗细，样式，颜色 */
        #box{
            width: 300px;
            border: 1px solid red;
        }

        h2{
            font-size: 16px;
            background-color: #8BC6EC;
            line-height: 30px;
        }

        form{
            background: #8BC6EC;
        }
        div:nth-of-type(1) input{
            border: 3px solid black;
        }
        div:nth-of-type(2) input{
            border: 3px dashed red;
        }
        div:nth-of-type(3) input{
            border: 3px dashed rgba(38, 166, 16, 0.45);
        }
    </style>

</head>
<body>

<div id="box">
    <h2>会员登录</h2>
    <form action="#">
        <div>
            <span>用户名：</span>
            <input type="text">
        </div>
        <div>
            <span>密码：</span>
            <input type="text">
        </div>
        <div>
            <span>邮箱：</span>
            <input type="text">
        </div>
    </form>
</div>

</body>
</html>